Joke about children vulgar

A little boy wakes up 3 nights in a row when he hears a thumping sound coming from his parents room. Finally one morning he goes to his mom and says, "Mommy, every night I hear you and daddy making noises and when I look in your bedroom you're bouncing up and down on him." His mom is taken by surprise and says, "Oh.. well.. ah.. well, I'm bouncing on his stomach because he's fat and that makes him thin again." And the boy says, "Well, that won't work!" His mom says, "Why?" And the boy replies, "Because the lady next door comes by after you leave each day and blows him back up."